Correlative high-resolution imaging of TMDs
Raman, SHG and PL imaging of an MoS2 flakeTwo-dimensional (2D) materials such as transition metal dichalcogenides (TMDs) arereceiving increasing attention due to their unique optical and electronic properties.
Their possible applications include the production of transistors, photo detectors, lightemitting diodes (LEDs) and photovoltaic cells. In order to produce high-quality devices,synthesis processes must be evaluated efficiently. Thus, non-destructive imaging
techniques are required for monitoring crystal properties and features such as grainboundaries, layer number, defect density, doping and strain fields.
The following measurement of CVDgrown mono-layer molybdenum disulfide (MoS2) illustrates the advantagesof correlative Raman, second harmonic
generation (SHG) and photoluminescence(PL) microscopy for investigating TMDs.
All measurements were performed at
the same area of interest using a WITecalpha300 microscope equipped with a532 nm laser for Raman and PL imaging
and a picosecond-pulsed 1064 nm laser for
SHG excitation.
In the white-light image (A), the contourof an MoS2 flake is visible, but no furtherfeatures can be discerned other than a few
bright spots caused by local bulk material.
In contrast, Raman, SHG and PL imaging
can reveal details about the material’s
structure and quality.
Second harmonic generation (SHG)
microscopy is a cutting-edge imaging
technique for the investigation of TMDs.
SHG is a nonlinear optical process that radiates a photon with twice the frequencyof the excitation photon and is sensitive tochanges in crystal orientation and symmetry [1, 2]. The high-resolution SHG intensity image (B) clearly visualizes the grainboundaries in the MoS2 flake (low SHG signal), as well as areas with increased signal.
In order to evaluate the optical resolutionof the measurement, a Lorentzian function
was fitted to the intensity profile along
the cross-section over one grain boundary.
The obtained full width at half maximumof about 630 nm is close to the diffraction
limited resolution of about 590 nm thatcan be achieved with 1064 nm excitation
and a 0.9 NA objective.
In addition, polarization-dependent SHG
measurements can identify the crystal
orientation and reveal strain fields [1, 3].
To this end, the excitation polarization isrotated while recording the intensity of
the SHG signal component that has thesame polarization as the incident light.
Due to the fully automated polarizer andanalyzer operation of the microscope used,polarization-dependent measurements
can be performed automatically within
seconds. Polarization series were recordedat three positions of the MoS2 flake (B)and represented in polar plots that depict
the recorded SHG signal as a function of
the excitation polarization angle (C). A
particularly symmetric pattern is observed
for the position marked in red, indicating
little strain in the crystal [3]. In contrast,
the other two positions show distinct
asymmetric patterns representing different strain levels.
Raman imaging can reveal strain fields incrystals and quantify the number of layers
in few-layer TMDs [4]. A typical Raman
spectrum of MoS2 shows two prominent
bands, the E2g band originating from anin-plane vibration and the A1g band originating from an out-of-plane vibration (D).
Their exact positions were quantified forevery image pixel by fitting a Lorentzianfunction and their respective frequencies
were color-coded in Raman images (E, F).
Grain boundaries are not visible in theRaman images, but some areas that sho-wed increased SHG intensity also show apronounced shift of the A1g and E2g bands.
In the Raman image of the A1g frequency,
rim effects are visible at the borders of theMoS2 flake, where the A1g band is the moststrongly red-shifted (F). The frequency of
the E2g band is particularly sensitive forstrain in the crystal [4, 5]. The corresponding Raman image revealed two areas ofstrong tensile strain, indicted by a pronounced red-shift of the E2g band (E).
Photoluminescence imaging can be used
for investigating strain, doping, defects
and layer number in TMD crystals [6, 7].
Intensity, wavelength and width of theMoS2 flake’s PL signal were fitted by a
Lorentzian function and color-coded in
the PL images (G, H, I). The PL intensity isincreased at the rim of the flake and close
to its center, while the spots that showed
increased SHG signal correlate with low
PL intensity (G). The rim effects are evenmore clearly visible from the pronounced
blue-shift of the PL wavelength around theborder of the MoS2 flake (H). The PL signal
is also sensitive to strain [6, 7] and the PL
wavelength shows a pronounced redshift in the same areas as the Raman E2g
frequency (E, H). This is also correlated withincreased width of the PL signal (I).
Correlative Raman, PL and SHG imaging
yields complementary and consistent
information for characterizing single-layer
TMD crystals by visualizing features of thecrystal structure, such as grain boundaries
or strain fields, without damaging thesample.

A: White-light image

B: SHG intensity

C: Polarization-dependent SHG

D: MoS2 Raman spectrum

E: Raman E2g frequency

F: Raman A1g frequency

G: PL intensity

H: PL wavelength

I: PL width

Correlative Raman, PL and SHG imaging of mono-layer MoS2. Image size: 50 µm x 50 µm with 200 nm per pixel for all SHG, Raman and PL images.

A: White-light image. B: SHG intensity image. Polarization series (C) were recorded at the three indicated positions. C: Polar plots of the SHG signal as afunction of the excitation polarization angle at the three positions indicated in B. The intensity refers to the SHG component that has the same polarization as the incident light. D: Raman spectrum of MoS2 on a silicon substrate. E-F: Raman images displaying the frequency of the E2g mode (E) and the A1gmode (F). G-I: Photoluminescence images representing the signal intensity (G), wavelength (H) and width (I).

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[6] S. Luo et al., Investigation of growth-induced
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